31 Search in a Binary Search Tree

    4
   / \
  2   7
 / \
1   3

  2     
 / \   
1   3

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
        
    }
}

class Solution {
    public TreeNode searchBST(TreeNode root, int val) {
    	// 递归终点，root为空说明不存在，root.val==val说明已找到
        if(root==null || root.val==val)
            return root;
        // 根据二叉搜索树的特性，比较目标值决定遍历方向
        if(root.val>val)
            return searchBST(root.left,val);
        else
            return searchBST(root.right,val);
    }
}